An electromagnetic induction type contactless power supply system is provided with contactless power supply device, which includes a primary coil, and a power reception device, which includes a secondary coil. When an electric appliance including a power reception device is set on a setting surface of a contactless power supply device, the contactless power supply device excites a power supply coil. This excites a power reception coil, which is arranged in the power reception device of the electric appliance, through electromagnetic induction. The power reception coil generates secondary power that is converted into DC power and supplied to a load of the electric appliance.
When a metal foreign object is present between the contactless power supply device and electric appliance (power reception device), the metal foreign object may be heated by induction when power is being supplied. Hence, the contactless power supply systems may include a metal detection device that detects a metal foreign object. When the metal detection device detects a metal foreign object, the contactless power supply device stops supplying power.
A conventional contactless power supply system includes a primary authentication coil, which is arranged in a contactless power supply device, and a secondary authentication coil, which is arranged in a power reception device, to exchange authentication signals between the contactless power supply device and the power reception device. The contactless power supply system uses the primary and secondary authentication coils to detect a metal foreign object (for example, WO 2011/036863).
In the above publication, magnetic coupling occurs between the primary and secondary authentication coils when a metal foreign object is present between the contactless power supply device and the power reception device, and the contactless power supply system uses this effect to detect a metal foreign object. In detail, when a metal foreign object is present, an electromagnetic induction signal of the secondary authentication coils responding to the excitation of the primary authentication coils is decreased in amplitude. The electromagnetic induction signal of the secondary authentication coil is provided to the primary authentication coils as a load modulation signal. The contactless power supply device detects a metal foreign object from the amplitude of the provided load modulation signal.